1. Field of the Invention
The present invention relates to a method of repairing damage of aircraft engine components made of weldable thermoplastic materials, wherein the damaged area, in particular cracks are welded using melted filler material which is the same weldable thermoplastic material as the material of the damaged component. The method is useful in particular in the area of maintenance services in aeronautics for extending the service time of thermoplastic components of aircraft engines while maintaining required mechanical strength parameters.
2. Discussion of Background Information
From the beginning in aircraft construction use of lightweight materials has been a priority, aiming to improve thrust-to-weight ratio. Besides metallic materials including high strength steel and alloys of metals of density lower than steel, such as aluminum and magnesium, the group of widely used materials are polymers and polymeric composites. As in many other areas of construction, initial use of polymers and composites thereof involved mostly thermosetting polymers, but since several years there is a clear and growing tendency to use thermoplastic polymers wherever possible. The thermoplastic polymers and some thermoplastic-elastomer composites are meltable and heat formable, allowing to obtain a wide variety of complex shapes by application of heat and pressure.
Among other components aircraft engines include elements made of plastics, such as in particular shielding panels made of thermoplastic material surrounding a jet engine fan. To assure reliable assembly to metallic body of the fan or other parts of the engine, most of these plastic components are provided with metallic bushings, typically made of aluminum alloys, embedded in the plastic material and designed to receive screws, bolts or other connecting means.
Some aircraft engine plastic components undergo high stress during engine operation. Factors to be taken into account include operation in broad temperature ranges, and varying dynamic stresses, such as between operation of a jet engine in a normal mode and in a thrust reverser mode. This may eventually lead to extensive cracking, often occurring close to the plastic body-metallic bushing interface or close to external edges of the elements. Such damages components have to be either replaced by new parts, which is expensive, or to be repaired. Due to safety reasons, which are extremely important in aeronautics (defined in numerous regulations and controlled by administrative bodies such as Federal Aviation Administration or European Aviation Safety Agency), the repaired parts have to meet strict technical standards, including in particular sufficient mechanical strength. Therefore, repairs of plastic parts of aircraft engines are generally practiced to much smaller extent than in any other field of technology and only with respect to relatively small cracks or damages.
Typically aircraft engine components made of plastics are repaired using composite tape or glue technique. This technique is generally capable to heal small cracks damages, but cannot be effectively used for heavy damages and intensive cracking. Therefore heavily damaged parts must be removed and replaced by expensive new parts. Furthermore, even slightly damaged parts, which have been repaired by glue technique do not show sufficient durability in further service operation, and therefore the applicability of this technique is limited to minor elements not exposed to high stress during operation.
On the other hand, welding of thermoplastic materials has been known for decades in various areas, mostly for joining together plastic elements to produce more complex shapes, e.g. difficult to obtain by injection moulding or joining together two different polymeric materials. Numerous plastic welding techniques are known, such as hot air welding (including in particular using welding rod, but also heat sealing or freehand welding), speed tip welding, extrusion welding, contact welding, hot plate welding, high frequency welding, induction welding, injection welding, ultrasonic welding, friction welding, spin welding, laser welding and solvent welding. The choice of specific technique is dependent on the actual task, that is the type of thermoplastic material(s), specific form of the parts to be joined, environment conditions and requirements (e.g. presence of flammable gases, limited access to the welding area), desired properties of the welded structure, presence of other materials (e.g. metallic elements, which might heat up too much during the process and negatively affect the final outcome), overall costs of the process, timing, availability of the respectively trained operators and required equipment, etc. Exemplary publications from this field discussing in detail various aspects of the welding of plastics such as PP (polypropylene), PE (polyethylene), PVC (polyvinylchloride), CPVC (chlorinated PVC), PVDF (polyvinylidene fluoride) and ABS (terpolymer: acrylonitrile-butadiene-styrene) include brochures entitled “Guidelines for welding thermoplastic materials” available at www.wegenerwelding.com/pdf/09_Guidelines.pdf and “Tipps un Tricks zum Schweissen von Thermoplasten” available at www.wegenerwelding.de/PDFs/schweissfibel.pdf.
In addition to manufacturing new parts of thermoplastic materials, welding techniques have been also known in various applications related to repair of damages plastic parts. For example, in WO 2006/009778 (A1), the entire disclosure of which is incorporated by reference herein, a method of repairing a discrepancy in a thermoplastic workpiece is disclosed, said method including excising the discrepancy by cutting, milling, or otherwise removing workpiece material surrounding the discrepancy so as to remove the discrepancy and form a cylindrical hole in the workpiece. The method further includes placing a cylindrical plug in the cylindrical hole and moving a friction stir welding tool around the circumference of the plug to weld the plug to the workpiece.
Further, in EA 201100480 (A1), the entire disclosure of which is incorporated by reference herein, a method of restoring articles from thermoplastic polymer material by a method of extrusion welding is disclosed. The method involves the steps of cleaning and mechanical surface preparation of an article to be restored, activation of the article surface by hot air heating with parallel heating of a thermoplastic polymer material to be welded, extruding of the welded material on an activated restoring surface through an extruder nozzle, cooling the article and subsequent mechanical surface treatment to the required parameters. This method requires that both the main and the welded material must have almost the same chemical composition and indices of melt plasticity.
US 2015/0001768 (A1), the entire disclosure of which is incorporated by reference herein, discloses a method of repairing damage in a thermoplastic component, wherein the damaged component is positioned between opposing corresponding die portions provided with a magnetic system for bringing the portions together in an arrangement corresponding to the shape of the undamaged component, selectively heating the component at the damage zone for a time sufficient to heal the damage by the die portions reshaping the component to its original, undamaged form.
However, in the field of maintenance of plastic parts of aircraft engines the repairs were limited only to small damages such as small cracks and were realized exclusively by glue technique. More seriously damaged plastic parts have been generally replaced by new ones, as there was no reliable method for maintaining the required parameters (in particular mechanical strength and stress resistance) in case of repairs of more extensive cracking. No publication known to the inventors disclosed attempts to test any other repair method of such thermoplastic parts and/or to measure the durability of such repaired elements. Therefore, there was a clear technical prejudice in the prior art, discouraging engineers from considering repair of more serious damaged areas of plastic parts of aircraft engines, in particular due to safety requirements which are much stricter than in other fields of application of such materials.